This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C xc2xa7119 from an application entitled earlier filed in the Korean Industrial Property Office on Dec. 4, 1996, and there duly assigned Serial No. 96-61664 by that Office.
1. Field of the Invention
The present invention relates to a still image compressing apparatus, and more particularly, to a still image compressing apparatus which can constantly maintain the compression rate of still images to within a predetermined range, and a method thereof.
2. Description of Related Art
Multimedia information is mainly comprised of a picture and sound. However, when this information is processed by pulse code modulation (PCM), the amount of data is excessive, which causes troubles in storage and transmission of the data. Accordingly, one of the prerequisites of multimedia is a technique of effectively compressing pictures and sounds. Thus, an international standard has been set to govern compression and multiplexing of pictures and sounds. In particular, JPEG (Joint Photographic Experts Group) is a standard developed for still image processing, such as in computer graphics.
A still image compressing apparatus based on JPEG has a discrete cosine transformer (DCT), a quantizer and an entropy encoder as basic components. The corresponding JPEG still image decoding apparatus includes an entropy decoder, a dequantizer and a reverse DCT as basic components. U.S. Pat. No. 5,629,780 to Andrew B. Watson, entitled Image Data Compression Having Minimum Perceptual Error, describes such a still image compressing apparatus and is incorporated herein by reference.
Typically, after discrete cosine transformation, the values of pixels irregularly distributed on a screen before transformation are likely to be concentrated toward the low frequency coefficients. Therefore, the compression of information can be performed with little noticeable loss of information by discarding the high frequency coefficients. One way of achieving this is quantization. Quantization is a process of dividing respective coefficients corresponding to each discrete-cosine transformed frequency by a predetermined quantization step size. Many terms become zero with an increase in quantization step size, and thus many high frequency terms are lost but the compression rate increases. In the decoding process, dequantization is performed by multiplying an entropy decoded image signal by the quantization step size.
In the still image compressing apparatus based on JPEG, a quantization step size used in the above-described quantization or dequantization process is provided in the form of the following standard table.
Typically, if there are many coefficients respectively corresponding to each discrete cosine transformed frequency having a high frequency component, the high frequency component increases and the compression rate is deteriorated, even after quantization by the quantization step size suggested by the quantization table. That is, when the quantization tables are used, the image quality is improved but the compression rate of a still image having many high-frequency components is lowered, and the compression rate of a still image having few high-frequency components is increased. Thus, the compression rate varies greatly according to the frequency components of the still image.
As described above, the compression rate of a still image by the JPEG method has a wide variation depending on the content of the still image. Therefore, the time needed to record, reproduce or transmit the still image varies greatly according to the content of the image to be compressed.
To solve the above problem, it is an object of the present invention to provide an apparatus for compressing a still image at a regular compression rate, which minimizes the variation of the compression rate by controlling high frequency components of a quantization table, and a method therefor.
To accomplish the above object, there is provided a still image compressing apparatus which constantly maintains a compression rate, the apparatus comprising: a luminance and chrominance signal generator which receives R, G and B signals of a still image and generates luminance and chrominance signals; a discrete cosine transformer which divides the image, represented by the luminance and chrominance signals, into square areas of a predetermined size, and decomposes each square area into coefficients for all the frequencies ranging from a mean value to a maximum frequency image component; a regular compression rate quantizer which divides each coefficient by a predetermined quantization step size, where coefficients corresponding to frequencies higher than a reference frequency are divided by the maximum possible step size; and an entropy decoder having a code scheme of allotting short-length codes to values having a high probability of occurrence among the quantized coefficients, and long-length codes to values having a low probability of occurrence among the quantized coefficients.
Also, to accomplish the above object, there is provided a method of constantly maintaining a compression rate, comprising the steps of: receiving R, G and B signals of a still image and generating luminance and chrominance signals; dividing the image, represented by the luminance and chrominance signals, into square areas of a predetermined size; discrete-cosine transforming each square area into coefficients for all the frequencies ranging from a mean frequency value to a maximum frequency value; dividing each coefficient by a predetermined quantization step size, where the maximum possible step size is used for coefficients corresponding to frequencies higher than a reference frequency; and allotting short-length codes to values having a high probability of occurrence among the quantized coefficients, and long-length codes to values having a low probability of occurrence among the quantized coefficients.